The ball on the TopspinPro is designed to be soft.  Tennis balls are typically graded as follows:

red – larger ball, very soft – up to 6 yrs typically
orange – regular size, soft – up to 8 yrs typically
green – regular size, less soft – up to 10 yrs typically
yellow – regular size, firm – everyone else

We use a “green” ball and it’s intentionally soft. The unit provides a lot of resistance anyway. In addition, you’ll notice the white plastic spindle passes through the ball – so there is actually a hole in the ball. It doesn’t have any air pressure.